Magnetic fluid or ferrofluid magnetic seals have been used both as exclusion seals, where there is no significant pressure difference on either side of the ferrofluid seal, or as multiple-stage seals, wherein a difference in pressure environment is involved between the different sides of the seal. Ferrofluid exclusion seal apparatuses are particularly useful in providing effective seals in computer-disc-drive systems and in other high-performance, precision spindle applications, while multiple-stage seals are particularly employed between a first low- and a second high-pressure environment, such as between a vacuum environment and an atmospheric environment.
Multiple-stage ferrofluid seals, such as described in U.S. Pat. No. 3,620,584, issued Nov. 16, 1971, typically are used to separate different pressure environments, such as a vacuum and an atmospheric pressure, with the pressure capacity of the seal being based on the number of the particular ferrofluid O-ring seal stages used. The multiple stages of the multiple-stage seal are formed through the use of knife or other edges on one or typically both pole pieces, or on the surface of the magnetically permeable shaft element underneath one or the other or typically both pole pieces, with the ferrofluid magnetically retained in a magnetic flux under each of the particular edges in the radial gap. In operation, the multiple-stage seal forms a plurality of separate, ferrofluid O-ring seals, with each O-ring seal defining, in an accumulative manner, a pressure capacity, typically with an air space between each of the multiple-stage seals, as well as an air space, as in the exclusion seal, between the first and second pole pieces beneath the annular permanent magnet.
Multiple-stage seals of U.S. Pat. No. 3,620,584 employ a common permanent-magnet source as a source of magnetic flux for all stages of the seal. However, multiple-stage seals also may be formed by employing a plurality of separate magnets, although in manufacture these seals tend to be much bulkier than wherein a single, common permanent magnet is employed.
It is desirable to provide improved, ferrofluid magnetic pressure-capacity seals, particularly where multiple-stage seals are to be employed in separating different pressure environments, and to provide a ferrofluid multiple-stage seal which has less of a tendency to air bursts; that is, to admit air into the lower-pressure environment and thus to provide a more stable, predictable and safer multiple-stage ferrofluid pressure seal apparatus.